1. Field of the Invention
The present invention relates to image display devices, and is applicable, for example, to a connection between a TFT substrate and a flexible printed circuit in a liquid crystal display device.
2. Description of the Background Art
A liquid crystal display device using thin film transistors (TFTs) includes a TFT substrate having TFTs arranged in a matrix, a counter substrate having counter electrodes, and a layer of liquid crystal sandwiched between the TFT substrate and the opposing substrate. The TFTs are formed in the vicinities of the intersections of a plurality of gate interconnections carrying TFT turning on/off signals and a plurality of source interconnections intersecting with the gate interconnections in plan view. One pixel is formed for each TFT.
With the TFT substrate and the opposing substrate placed on each other, the periphery of the TFT substrate projects beyond the periphery of the opposing substrate. In the projecting area of the TFT substrate, a driver IC is mounded with an anisotropic conductive film (ACF) interposed therebetween. The driver IC is a circuit that controls the turning on/off of the TFTs. The ACF is a connecting material that has conducting, insulating, and adhesion functions, with conductive particles dispersed in an adhesive film.
A circuit board for controlling the driver IC is provided separately from the TFT substrate, and the TFT substrate and the circuit board are connected through a flexible printed circuit (FPC). The TFT substrate and the FPC are connected through an ACF. The connection between the TFT substrate and the FPC is made in the projecting area of the TFT substrate.
In this structure, driver IC control signals, power supply, and the like are supplied from the circuit board to the driver IC through the connection between the TFT substrate and the FPC.
An example of specific structure of the connection between the TFT substrate and the FPC in a liquid crystal display device is disclosed in Japanese Patent Application Laid-Open No. 9-179505 (1997), for example.
According to conventional techniques, a plurality of first terminals made of a thin film of metal are formed on the TFT substrate. Specifically, the first terminals are arranged like stripes on the TFT substrate. On the other hand, the FPC includes a film of polyimide and second terminals made of copper foil on the surface of the polyimide film. The thickness of the first terminals is smaller than that of the second terminals.
The first terminals are connected respectively with the second terminals to make the connection between the TFT substrate and the FPC. The ACF is placed between the first terminals and the second terminals. More specifically, conductive particles in the ACF exist between the first terminals and the second terminals and the presence of the conductive particles electrically connects the first terminals and the second terminals.
The second terminals may include high-potential terminals supplied with relatively high potential and low-potential terminals supplied with relatively low potential, which are arranged adjacent to one another.
In this structure, as mentioned above, control signals and power supply are supplied from the circuit board to the driver IC on the TFT substrate through the connection between the second terminals and the first terminals.
Suppose that a liquid crystal display device having this connection structure is operated in a hot and humid environment. Then, water will penetrate the ACF to cause the following phenomenon in thin first terminals that are connected to high-potential terminals. That is, in these first terminals, the constituent metal changes into positive ions and erodes. The eroded first terminals cause disconnection.
The thickness of the first terminals is around 1 μm or less, while the thickness of the second terminals is about 15 to 35 μm. Therefore, the thinner first terminals are more severely affected by the erosion. Also, the first terminals connected with high-potential terminals are more likely to be eroded when the potential difference between the high-potential terminals and the low-potential terminals becomes larger.